[Test Proxy] Add fixture to automatically start/stop Docker container

[mccoyp]

This adds a session-level fixture, test_proxy, that can be used to start the proxy Docker container before recorded tests run and shut it down after all tests in the pytest session are run. It also exposes two helper methods used by the fixture, start_test_proxy and stop_test_proxy, that can be called directly to manually start or stop the proxy container.

The container needs some time to warm up before it'll accept incoming requests, so the start_test_proxy method pings a proxy endpoint with GET requests until receiving a 200 (or until a 10-minute timeout is hit).

Usage example (that will be added to the migration guide soon)
In a package's conftest.py, make the fixture name a parameter to any proxy-dependent, session-level fixtures:

import os
from devtools_testutils import add_general_regex_sanitizer, test_proxy

@pytest.fixture(scope="session", autouse=True)
def sanitize_account_name(test_proxy):
    add_general_regex_sanitizer(value="fakeendpoint", regex=os.envrion["TABLES_STORAGE_ACCOUNT_NAME"])

This will make sure the the container startup script is run before any sanitizers are registered (otherwise, we'd get errors). If no sanitizers are being registered with session-level fixtures, a dummy fixture can be added to the package's conftest.py instead, that just invokes the test_proxy fixture:

from devtools_testutils import test_proxy

@pytest.fixture(scope="session", autouse=True)
def start_proxy(test_proxy):
    return

[scbedd]

We need to handle PROXY_MANUAL_START here. Once we do that this is good to merge.